In the evolution of commercial fabrication of semiconductor wafer, larger and larger wafers are processed in bigger and bigger batches. Such processing has pushed the performance envelope of processing equipment, as well as that of the wafer handling and carrying mechanisms needed to move, transport, and retain the wafers during processing.
In many chemical and thermal processing operations, it is often necessary to hold the wafers in precise positions during various processing steps. Relatively large and complex structures such as "boats" or "towers" are typically employed to that end. One example of such a structure is described in U.S. Pat. No. 5,492,229 to Tanaka et al. The Tanaka et al. patent is directed to a vertical boat for holding a plurality of semiconductor wafers. The boat includes two end members and a plurality of support members. In one embodiment, the support members are formed from pipe members cut vertically to provide a long plate member having a cross section of a quarter-circular arc. In another embodiment, the support members are formed from pipe members cut vertically to provide a long plate member having a cross section of a semicircular arc. The Tanaka et al. patent lists as potential materials for its boats the following: silica glass, silicon carbide, carbon, monocrystal silicon, polycrystal silicon, and silicon carbide impregnated with silicon. The various components are to be welded together if made from silica glass; otherwise, "they may be assembled in a predetermined manner".
The theoretical advantages provided by pure silicon structures are well known. Conventional towers and boats are typically made from quartz, which introduces contamination and becomes unstable at higher temperatures. By fabricating wafer holding structures from the same materials as the wafers themselves, the possibility of contamination and deformation would be minimized. The structure would react to processing temperatures, conditions, and chemistry in exactly the same way that the wafers would, thus greatly enhancing the overall effective useful life of the structure.
Unfortunately, standard assembly of silicon structures in a "predetermined manner" as set forth in Tanaka et al. is one of the reasons that pure silicon has not gained wide acceptance as a material for structures such as boats and towers. The difficulties of working with monocrystalline and polycrystalline silicon have led to the development of structures such as that shown in Tanaka et al. When considering monocrystalline silicon as the material of choice in such structures, the Tanaka et al. patent fails to describe the connections between the support members and the end members. The only specifically described method of fabricating support structures involves cutting extruded tubular members. Such support structures are inherently less stable than those made from more traditional and easily-worked materials such as quartz or silicon carbide.
Silicon is perceived as being extremely fragile and difficult to weld. Due to these perceptions, known silicon structures are widely believed to be delicate at best, and unreliably flimsy at worst. Consequently, they have failed to receive broad commercial acceptance.
Furthermore, due to its molecular structure, blanks extruded from crystalline silicon have a distinct "grain" running generally longitudinally through the blank. Silicon blanks are usually cut laterally, across the grain, using a scroll saw. Unfortunately, when used to make longitudinal cuts, conventional cutting techniques tend to split silicon blanks along the grain, thus ruining the blank.
It can be seen that the need exists for a method of fabricating monocrystalline and polycrystalline silicon structural members for use in the manufacture of semiconductor wafers and the like that will eliminate the disadvantages of known silicon structures while retaining the advantages of silicon as a structural material